1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor (HBT) using a semiconductor material for its emitter region, which is larger in band gap than that used for its base region.
2. Description of the Related Art
As a result of their excellent high-frequency and switching characteristics, heterojunction bipolar transistors are considered very promising as microwave transistors and high-speed logic transistors.
In general, the speed performance of bipolar transistors is represented by the cutoff frequency f.sub.T. The cutoff frequency f.sub.T of npn transistors is represented by the reciprocal of the transit time of electrons in the transistors. Thus, to achieve a high cutoff frequency, it is necessary to reduce the electron transit time. The transit time .tau. of electrons in a transistor is given by the sum of an emitter-charging time .tau..sub.E, a base transit time .tau..sub.B, and a collector transit time and collector-charging time .tau..sub.C. .tau..sub.B is independent of current density and is virtually constant, while .tau..sub.E decreases with increasing current density, and will be below 1 psec under the conditions of high current density in excess of 10.sup.5 A/cm.sup.2.
In the case of .tau..sub.C, on the other hand, the collector charging time, which is based on the collector junction capacitance, can be decreased sufficiently with an increase in the current density, but the collector transit time cannot sufficiently be decreased.
In particular, in the case of conventional general structures having p+(base)-n.sup.- (collector) junctions, electrons run through a depletion layer at a low saturation velocity of 6 to 8.times.10.sup.6 cm/sec. Thus, if the depletion layer increases in width, then .tau..sub.C would almost simply increase in proportion to the width of the depletion layer. For this reason, .tau..sub.C would have the most effect on f.sub.T.
Next, the collector transit time as explained above will now be described below in more detail.
FIG. 1 shows a conduction band in the base-collector region of a heterojunction bipolar transistor with a conventional structure of n emitter - p.sup.+ - n.sup.- collector - n.sup.+ collector. It should be noted that the conduction band refers to the -valley. In the p.sup.+ -n.sup.- junction, the conduction band abruptly bends at the interface between the base region and the collector region. In the drawing a dotted line denotes the L-valley. Electrons moving from the base region into the collector region instantly transmit from the -valley to the L-valley by intervalley scattering therein. Because of the intervalley scattering the wave number vector has its direction disturbed, reducing the velocity of electrons.
In order to clarify the foregoing, the results of the calculations of electron-drift-velocity distribution, carried out by the present inventors using the Monte Carlo simulation, are shown in FIG. 2. As can be seen from FIG. 2, an overshoot of the electron velocity occurs near the base-collector junction. However, the distance within which the electrons are subject to the overshoot is below 500 .ANG.. The electrons will run at a slow saturation speed of 6 to 7.times.10.sup.6 cm/sec through the remainder of the depletion layer.
Various attempts have been made to reduce the collector transit time of electrons. One such attempt involves a structure in which a p.sup.- base is provided between a p.sup.+ base and an n.sup.+ collector ("A Proposed Structure for Collector Transmit-Time Reduction in AlGaAs/GaAs Bipolar Transistors"; C. M. Maziar et al. IEEE Electron Devices Lett. Vol. EDL-7, No. 8. 1986. p483). With this structure, to be sure, the velocity overshoot becomes easy to occur as compared with the conventional structures. Since the p.sup.- -n.sup.+ junction corresponds to the p.sup.+ -n.sup.- junction, however, the inclination of the conduction band will be made abrupt as shown in the conduction band diagram of FIG. 1 for conventional structures. Therefore, noticeable reduction of the collector transit time may not be expected. Furthermore, this structure has a disadvantage that breakdown voltage is low.
A heterojunction bipolar transistor with a structure of p.sup.+ -p.sup.- (base)-n.sup.- n.sup.+ (collector) is disclosed in "GaAlAs/GaAs Hetero Junction Microwave Bipolar Transistor"; H. Beneking et al. Electronics Lett. Vol. 17, No. 8, 1981 p301. With this structure, since the p.sup.- base is completely depleted in operation of the transistor, and hence the conduction band diagram is similar to that of FIG. 1, the collector transit time of electrons will not be reduced.
With conventional heterojunction bipolar transistors, the velocity overshoot effects seen remarkably in compound semiconductors have hardly been employed usefully. The collector transit time would be governed by a slow saturation velocity in a high electric field region. Therefore, the merits of heterojunction bipolar transistors could not be used usefully in case that a collector depletion layer is large in width.
There is a physical quantity of maximum oscillation frequency f.sub.MAX to represent the speed performance of bipolar transistors. The maximum oscillation frequency can be described as EQU f.sub.MAX =(f.sub.T /8.pi.R.sub.B C.sub.C).sup.1/2
where f.sub.T stands for the cutoff frequency, C.sub.C the collector junction capacitance, and R.sub.B the base resistance.
To raise the maximum oscillation frequency f.sub.MAX it is necessary to raise the cutoff frequency f.sub.T and reduce the collector capacitance C.sub.C. If the width of a depletion layer is increased in order to reduce the collector capacitance C.sub.C, then the collector transit time will be increased and hence the cutoff frequency will be lowered. For this reason a technique is highly desired which enables the cut-off frequency and the maximum oscillation frequency to be raised while securing the desired width of the collector depletion layer.